Many automated chemical analysis instruments include tubular transfer means or the like for conducting samples into other instrument elements. Various bubble sensors are known in which a photodetector such as a photodiode or phototransistor diametrically opposed (180.degree.) to a light source such as a light emitting diode (LED) to detect air bubbles in fluid flowing through the lumen of a tube. Bubbles are detected by such devices as a consequence of the reduction in optical coupling caused by the presence of bubbles in fluid flowing through the tube lumen.
There appears, however, to be a paucity of prior art teaching of devices useful to detect the presence or absence of fluids flowing through tube lumens and also the presence of bubbles in such flowing fluids.